Random copolymers of vinyl aromatic and conjugated diene monomer (e.g., poly(styrene-co-butadiene)), are industrially useful. For example, these random copolymers are often used in the manufacture of tire components such as tire treads.
Those skilled in the art appreciate that these copolymers are not entirely random and that blocks of vinyl aromatic mer units, such as styrene mer units, exist in the polymer chain. And, it is believed that the presence of these blocks, particularly those of a smaller size, can be advantageous to the performance of the polymer. Accordingly, synthetic techniques have been developed to control the presence and size of these vinyl aromatic blocks.
The vinyl aromatic blocks that are believed to be advantageous are referred to as microblocks. These microblocks generally include from about 2 to about 12 repeating vinyl aromatic mer units with the preferred range generally including from about 3 to about 10 repeating mer units. The amount of these microblocks within a given polymer can be expressed as a weight average based upon the weight of all styrene within the copolymer. For example, a copolymer having 10 percent microblock content is a copolymer wherein 10 weight percent of the styrene mer units of the copolymer are located within microblocks. The balance of the styrene mer units may include completely random styrene mer units or styrene mer units located in blocks that are larger than microblocks.
Nuclear magnetic resonance (NMR) has been employed to quantitatively determine the styrene microblock content of copolymers. See, for example, Determination of Sequence Distribution in Styrene-Butadiene Copolymer i. 1H-NMR Study of Styrene Oligomers, Tanaka, et al. 685 RUBBER CHEMISTRY AND TECHNOLOGY VOL. 54. This method has advantageously provided quantitative analysis of styrene microblock content with high levels of accuracy and reproducibility. While this is a useful technique, especially within research and development settings, the technique is not as useful for manufacturing quality control (QC) because QC labs are often not equipped with personnel and/or equipment needed to perform NMR analysis. There is, however, a need for QC analysis of styrene microblock content that has a high degree of quantitative precision and reproducibility.